Dealing with numeric obfuscation in malicious scripts
Deobfuscating malicious scripts often involves dealing with numbers to be converted into strings, with or without calculations.
Like this PowerShell script:
This script will convert a list of numbers (100, 105, …) into a list of characters (converting each number into its corresponding ASCII value), join them together into a string and execute the command inside this string.
I have to do this kind of analysis so frequently, that a couple of years ago, I made a simple tool to help me: numbers-to-string.py.
This tool takes a text file as input, extracts numbers per line of text and produces a string for each line with at least 3 numbers. Here this tools deobfuscates the above PowerShell script:
When numbers-to-string is used without argument, it converts each number to its ASCII value (if an error occurs, because a number is too large for example, the line is silently skipped).
Often, scripts with obfuscated commands will be a bit more complex than this simple example. Like the following script, where a mathematical operation is performed on each number, before it is converted:
Converting this with numbers-to-string does not yield the desired result:
This can be fixed by providing numbers-to-string with a mathematical expression as argument to be applied for each number. It's clear that in the script, 3 is added to each number ($_ + 3). By providing expression "n + 3" to numbers-to-string, 3 will be added to each number (n) before it is converted to its ASCII value:
With this expression, the obfuscated command is properly decoded, but notice the unprintable character at the end of the decoded string:
This is because of the conversion of number 3 (from $_ + 3). It's not part of the encoded command, but nevertheless, it has been converted by numbers-to-string. One way to avoid this, is to use option --end to instruct numbers-to-string when it should stop looking for numbers, like this:
With option --end Foreach, numbers-to-string will stop looking for numbers when it encounters the string Foreach.
Didier Stevens
Senior handler
Microsoft MVP
blog.DidierStevens.com DidierStevensLabs.com
Comments
Anonymous
Dec 3rd 2022
10 months ago
Anonymous
Dec 3rd 2022
10 months ago
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Dec 26th 2022
9 months ago
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Dec 26th 2022
9 months ago
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Dec 26th 2022
9 months ago
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Dec 26th 2022
9 months ago
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Dec 26th 2022
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Dec 26th 2022
9 months ago
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Dec 26th 2022
9 months ago
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